There is currently a need for a better means of assessing the vitality of embryos for transplantation into the would-be mother in IVF clinics. The general standard has been to assess the extent of development at 3 days and use this as a predictor of the likely success of subsequent development and hence the pregnancy. The unreliability of this assessment has led an increasing number of IVF clinics to culture embryos until the blastocyst stage (6 days) so that apparently healthy blastocysts can be used directly. Not only is this approach costly but there are inevitable risks from such long term in vitro culture to the blastocyst stage.
The early events in an embryo following fertilization of an egg have been characterized. In mammalian zygotes, sperm entry is linked with reorganization of the cytoskeleton, cortical granules7, endoplasmic reticulum8 and mitochondria9. Sperm entry causes changes in zygote shape: formation of the second polar body (PB); protrusion of the fertilization cone (FC) above sperm chromatin17, 18; and flattening of the zygote along the axis that bisects the FC19. Mammalian fertilization also triggers oscillations of free cytoplasmic Ca2+ between completion of meiosis and entry into interphase of the first embryonic cell cycle10. The Ca2+ oscillations regulate not only events immediately following fertilization11-14, but were also reported to affect aspects of post-implantation development12, 15, 16. Both functional actin cytoskeleton and correct pattern of Ca2+ transients (especially the total time of Ca2+ elevation) have been reported to be crucial for development12, 15, 16, 37-39.
Cytoplasmic movements triggered by sperm entry have been described in some organisms. In sea urchins, ascidians and frogs, for example, they are known to be associated with successful progress of developmental events, such as the establishment of embryo polarity1-6. Some cytoplasmic motions have teen detected in the fertilized mouse egg20.
Despite this characterization of the events occurring in the embryo immediately after fertilization, assessing the extent of development of the embryos following a period of culture remains the preferred and standard method of assessing embryo quality.